Measurement of the decay times of organic scintillators

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A N D M E T H O D S 52 (i967) 2 9 3 - 2 9 7 ;

INSTRUMENTS

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PUBLISHING

CO.

MEASUREMENT OF THE DECAY TIMES OF ORGANIC SCINTILLATORS* J. KIRKBRIDE, E. C. YATES and D. G. C R A N D A L L

EG&G, Inc., Santa Barbara Division, Goleta, California, U.S.A, Received 24 February 1967 This paper describes a high-speed version of the monophoton technique for measuring the scintillation light decay as a function

of time for organic scintillators. Experimental results are given for a wide range of commercially available scintillators.

1. Introduction The well-known monophoton technique for measuring the decay of light intensity 1' 2) was first developed by Bollinger and Thomas 3) to obtain decay information spanning five orders of magnitude. The resolution of their system was 11.3 nsec fwhm. Koechlin 4) used the same method to achieve a 0.6-nsec time resolution, but the data spanned only one decade. The present measurements have a system resolution of 2 nsec fwhm and span five decades information. These conditions are adequate to determine the decay constant for the first decade of decay to an accuracy of 0.1 nsec. One disadvantage of the monophoton technique has been the slow rate at which data could be accumulated; for example, previous systems took 20 h to collect a single decay curve of 100000 counts. This paper describes the use of 100-Me/see logic modules to produce

a system that can collect a decay curve of 100 000 counts in 20 min or less. Also presented is an extensive set of decay measurements on commercially available scintillators taken with this fast equipment.

/ FLUOR

2. Apparatus The method for measuring a scintillator decay curve is briefly as follows. A 65Zn gamma source produces about 5000 scintillations/see in the cylindrical fluor, which is viewed at each end by a photomultiplier (PM I and PM II). Any single scintillation is seen fully and directly by PM I, and the anode output of PM I determines the "start" of the scintillation. The scintillation is also viewed by PM 11 through a light attenuator. The attenuation of light directed toward PM II is so made that the probability that it will detect the light flash is considerably less than unity. Thus, * Supported by the Atomic Energy Commission through the Lawrence Radiation Laboratory, Livermore, California. PM ANODE

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Fig. 3. Experimental decay curves for commercially available scintillators. when an event is observed in PM II, it will usually be the result of a single visible photon being detected. One then determines the average shape of the scintillation pulse for a large number of events by measuring the probability of detecting a visible photon in PM II as a function of time, with zero time being specified by PM 1. The scintillators examined in this program were aromatic plastics or liquids; the plastics were machined and polished,~]" x ½" scintillators, and the liquids were encapsulated in 1" x 1" glass cylinders. The scintillators, which are kept at 20 ° C by resistive heaters, are directly coupled through a lucite light pipe to PM I, which is kept at room temperature. PM II, which is cooled to 0°C to reduce thermal noise views the scintillator

through the attenuator (a neutral-density optical filter) and a 1" aperture. The radioactive source used in these measurements was 200 mCi of 65Zn (1.114 MeV gammas) and it was placed at the side of the scintillator. Block diagrams of the processing electronics are shown in figs. 1 and 2; most of the units are EG&G's 100-Mc/sec modules. The anode of PM I is fed directly through a 50-ohm cable to the fast single-channel analyzer, which has the function of selecting the upper 60% portion of the Compton distribution of scintillations and providing a fast, jitter-free, start pulse. Similarly, the anode of PM [I is fed directly into another fast single-channel analyzer, which selects the events caused by the release of single-channel photoelectrons from the PM ]I photocathode. The time

THE

DECAY

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interval between the start and stop pulses is converted into a pulse height by the time-height converter (THC), which is usually set with a maximum range of 300 nsec. The T H C output is sorted in the 400-channel pulseheight analyzer, giving the desired time profile of the scintillation intensity. Four sealers provide the following counts, which are required for data corrections: Ns = number of starts to T H C ; N c = number of start pulses followed within one T H C range (300nsec) by any PM II pulse crossing first discriminator threshold; NA = totalnumberofpulsesthatpulseheightanalyzer accepts for analysis; Nsw = number of stops delivered to THC.

2.1. DATA ANALYSIS The raw data are proce3sed by computer to take account of deadtime in the stop channel, slight nonlinearities in the time-to-height system, and random coincidences where necessary. The deadtime correction, similar to that developed by Bollinger and Thomas, is given by N~=N i l-{Nc

NsNA

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where

N[ = corrected count in channel i, N i = observed count in channel i.

,

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J. KIRKBRIDE et al.

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Fig. 5. Arrangement to measure the system time response. The s m o o t h i n g o f the results in the Iong-tail components was performed by grouping channels. 2.2. SYSTEMRESOLUTIONFUNCTION Since the scintillators studied in this report have initial decay times that are comparable to the resolving time o f the system, it is i m p o r t a n t that the system resolution function be accurately determined. Fig. 5 shows the experimental arrangement for measuring the system response function. The test scintillator is coupled directly, as usual, to P M I; and a 1" x 1" lucite (~erenkov radiator is m o u n t e d on P M II. The 65Zn source is replaced by a 6°Co source that emits two coincident g a m m a rays (1.17 and 1.33 MeV). The light output o f the (~erenkov radiator due to a 6°Co g a m m a ray is so low that an average of less than one visible p h o t o n per event is seen by P M I I as a scintillator with a zero decay time. The start-channel resolution, which depends on the light o u t p u t o f the test scintillator, is included in the measurement, as are the effects occurring in the stop channel that are due to detecting single photons. Measured in this manner, the system response function is between I and 2 nsec. It can be varied by varying the focus voltage o f P M lI, which controls the area o f the p h o t o c a t h o d e f r o m which photoelectrons are collected. I n practice, a focus voltage is chosen that minimizes a minor effect attributed to light feedback in P M I I ; this focus voltage results in a practical system resolution o f 2 nsec. The m i n o r effect is a small, justdiscernible b u m p occurring about 25 nsec after the m a i n peak on the decay curve. In principle, the system time resolution should vary f r o m scintillator to scintillator. Scintillators o f different

TABLE 1

Scintillator decay time (r) for the first decade of decay. State

Manufacturer

C C P P P P P P P P P

TE TE TE TE PC PC PC II NE NE NE NE NE NE PC NE NE NE NE NE NE NE NE NE NE NE NE NE NE NE NE NE NE NE NE NE NE NE

P P

P L L L L L L L L L L L L L L L L L L L L L L L L

Scintillator Anthracene Stilbene Naton 11 Naton 136 Pilot A Pilot B Pilot Y SC 700 NE 102 NE 103 NE 104 NE 104A NE 150 NE/60 Liquifluor NE 211 NE 213 NE 218 NE 219 NE 220 NE 223 NE 224 NE 225 NE 226 NE 227 NE 230 NE 231 NE 311 NE 311A NE 312 NE 313 NE 314 NE 315 NE 316 NE 317 NE 318A NE 321 NE 323

r (nsec) 29.3 4.68 2.60 1.87

2.96 1.90

3.09 4.86 2.51 11.14 1.92 1.88

2.17 2.27 2.35 2.61 3.71 3.90 8.97 3.84 7.09 2.73 5.82 3.28 17.02 2.98 2.76 3.84 3.66 4.47 4.01 2.42 4.60 4.03 20.21 4.06 15.70 3.83

C = Crystal; P = Plastic; L = Liquid. TE = Thorn Electronics, PC = Pilot Chemicals; II = Isotopes, Inc.; NE = Nuclear Enterprises. organic structure should differ in the rate of p h o t o n emission in the rise time, and this should cause differences in the jitter on the start p h o t o n multiplier (PM I). I n practice, no significant time resolution differences (due to this cause) have been detected, as indicated by the similarity o f the rise times for all the fluors examined (figs. 3 and 4). 3. R e s u l t s

The fast electronics, figs. 1 and 2, permitted a wide range of scintillator decays to be measured, as listed in table 1 and shown in figs. 3 and 4. The present measure-

THE DECAY TIMES OF O R G A N I C S C I N T I L L A T O R S

ments, which were made with a system resolution of 2 nsec fwhm, span more than three decades of decay information for the first 100 nsec of decay. The region of the decay curve between 70- and 7-percent of the peak value is found to vary exponentially for most scintillators. The decay time for this region was determined to an accuracy of _+ 0.1 nsec and is listed in table 1. The decay of light intensity from most of the scintillators studied can be thought of as having two components. The first decade of decay is very nearly exponential, with 1/e decay times, mainly between 2 and 5 nsec. This part of the decay is followed by a long nonexponential tail. Both the prompt and tail parts of the decay are due to light from the lowest excited singlet, Sx state in the scintillator, but there are two origins of these S 1 states. First, there is direct excitation of upper excited molecular singlet states followed by internal conversion to the lowest singlet state, which then gives the prompt component of the scintillation. Secondly, there is the formation of excited triplet states formed mainly from the rapid recombination of ions. These triplet states, during diffusion, form triplettriplet pairs that can decay with the formation of an $1

297

level, again causing light emission. This process is mainly responsible for the long-tail component, for which a theory has been developed by King and VoltzS). The times for the emitted light to reach a maximum are very short for all the scintillators studied, and are masked by the response time of the system. However, measurements of the rise times of scintillators have been reported by McGuire and Palmer6), who used another technique. References 1) R. L. McGuire, E. C. Yates, D. G. Crandall and C. R. Hatcher, IEEE Trans. Nucl. Sci. NS-12 (1965) 24. 2) E. C. Yates and D. G. Crandall, IEEE Trans. Nucl. Sci. NS-13, no. 3 (1966) 153. 3) L. M. Bollinger and G. E. Thomas, Rev. Sci. Instr. 32 (1961)

1044. 4) y. Koechlin and A. Raviart, Nucl. Instr. and Meth. 29 (1964) 45. 5) T. A. King and R. Voltz, Proc. Royal Soc. A289 (1966) 424. B)R. L. McGuire and R. C. Palmer, IEEE 13th Nucl. Sci. Symp. (1966).